1. Field of the Invention
The present invention relates in general to a braking system wherein fluid pressures in two brake cylinders are increased by distributing pressurized fluid delivered from a pump to the two brake cylinders, and more particularly to techniques for optimizing rates of increase of the fluid pressures in these two brake cylinders.
2. Discussion of the Related Art
A braking system for a motor vehicle may be required to electrically control the fluid pressures in brake cylinders for wheels of the vehicle. For example, the electrical control of the fluid pressures in the wheel brake cylinders is required to effect anti-lock control of the wheel brake cylinder pressures so as to prevent or reduce locking tendencies of the wheels upon brake application to the vehicle, or effect "braking effect" control of the wheel brake cylinder pressures so that the actual braking effect as represented by actual deceleration of the vehicle by brake application coincides with a value desired by the vehicle operator, irrespective of fluctuating factors such as a variation in friction coefficient of friction members used in hydraulically operated wheel brakes.
A vehicle braking system may have a pressure application sub-system which includes brake cylinders for a left and a right wheel of the vehicle. An example of this kind of vehicle braking system is a braking system of X-crossing or diagonal type having two pressure application sub-systems one of which has brake cylinders for a front left wheel and a rear right wheel, and the other of which has brake cylinders for a front right wheel and a rear left wheel.
JP-A-63-34272 discloses one example of a motor vehicle braking system which is capable of electrical control of the wheel brake cylinder pressures as described above and which has a pressure application sub-system as also described above. This braking system includes a reservoir, a pump alternately turned on and off to pressurize a working fluid received from the reservoir so as to intermittently deliver the pressurized fluid, two brake cylinders to which the pressurized fluid delivered from the pump is distributed, an electrically operated pressure control device connected to the reservoir, pump and the two brake cylinders, and a controller for electrically controlling the electrically operated pressure control valve to electrically control the fluid pressures in the two brake cylinders.
The assignee of the present invention developed a braking system as one form of the braking system described just above. This braking system is a diagonal or X-crossing type braking system for a four-wheel motor vehicle, which has two mutually independent pressure application sub-systems connected to respective two pressurizing chambers of a master cylinder. Each of the two pressure application sub-systems includes (a) a reservoir, (b) a pump which is alternately turned on and off to pressurize a working fluid received from the pump so as to intermittently deliver the pressurized fluid, (c) a pump passage which is connected to a delivery port of the pump and which has two branch passages, (d) two wheel brake cylinders which are connected to the ends of the respective branch passages of the pump passage and to which the pressurized fluid delivered from the pump is distributed, (e) a flow restrictor provided in each of the branch passages, (f) a electrically operated pressure control device connected to the reservoir, pump and two wheel brake cylinders, and (g) a controller which is operated upon brake application to the vehicle, to electrically control the electrically operated pressure control device for electrically controlling pressures in the two wheel brake cylinders, so as to prevent locking tendencies of the wheels corresponding to the wheel brake cylinders. It is noted that the reservoir is used commonly for the two pressure application sub-systems.
A further research by the inventor of the present invention on the braking system developed by the assignee of the present invention revealed the following fact:
In the developed braking system, the pressurized delivered from the pump is distributed to the two wheel brake cylinders through the flow restrictors provided in the branch passages of the pump passage. In an anti-lock control pressure operation of the braking system, the pump is held operated so that the pressurized fluid from the pump can be supplied at any time to the wheel brake cylinders. In some running condition of the vehicle, there exists a difference between the fluid pressures in the two wheel brake cylinders. For example, such a pressure difference exists when the vehicle is running on a road surface whose left and right areas have different friction coefficient values. While the delivery pressure of the pump is higher than the higher one of the pressures in the two wheel brake cylinder (hereinafter referred to as "high-pressure brake cylinder and low-pressure brake cylinder"), the pressurized fluid delivered from the pump is distributed to the high-pressure and low-pressure wheel brake cylinders, whereby the pressures in the high- and low-pressure brake cylinders are simultaneously increased.
When there exists a relatively large pressure difference between the two wheel brake cylinders, however, the rate of increase of the pressure in the high-pressure brake cylinder tends to be low. That is, when the pump is operated with a relatively large pressure difference of the two wheel brake cylinders, the delivery pressure of the pump is considerably higher than the pressure in the low-pressure brake cylinder, but is only slightly higher than the pressure in the high-pressure brake cylinder. Consequently, it is difficult for the pressurized fluid delivered from the pump to be fed into the high-pressure brake cylinder, and the amount of the pressurized fluid distributed from the pump to the high-pressure brake cylinder tends to be insufficient. As a result, the rates of increase of the pressures in the two wheel brake cylinders cannot be sufficiently equalized. Thus, the braking system suffers from poor controllability of the pressures in the two wheel brake cylinders.